Suppression and enhancement of decoherence in an atomic Josephson junction

Abstract

We investigate the role of interatomic interactions when a Bose gas, in a double-well potential with a finite tunneling probability (a 'Bose-Josephson junction'), is exposed to external noise. We examine the rate of decoherence of a system initially in its ground state with equal probability amplitudes in both sites. The noise may induce two kinds of effects: firstly, random shifts in the relative phase or number difference between the two wells and secondly, loss of atoms from the trap. The effects of induced phase fluctuations are mitigated by atom-atom interactions and tunneling, such that the dephasing rate may be suppressed by half its single-atom value. Random fluctuations may also be induced in the population difference between the wells, in which case atom-atom interactions considerably enhance the decoherence rate. A similar scenario is predicted for the case of atom loss, even if the loss rates from the two sites are equal. We find that if the initial state is number-squeezed due to interactions, then the loss process induces population fluctuations that reduce the coherence across the junction. We examine the parameters relevant for these effects in a typical atom chip device, using a simple model of the trapping potential, experimental data, and the theory of magnetic field fluctuations near metallic conductors. These results provide a framework for mapping the dynamical range of barriers engineered for specific applications and set the stage for more complex atom circuits ('atomtronics'). Figure 4. Decoherence induced by loss. A loss rate g = J 0.08 loss is applied, such that after = t J 10 only ∼45% of the atoms are left in the trap. During this time the coherence drops due to interactions. We have used N=50 atoms in the numerical simulation, showing two curves, for onsite interaction strengths = U J 0.2 [u = 10] and U=J [u = 50]. (a) coherence as a function of time. (b) instantaneous rate of decoherence, showing oscillations with half the Josephson period w = ( J 3.32 J and J 7.14 for the two curves).

Robust spatial coherence5μmfrom a room-temperature atom chip

“…Theory predicts that even if the chemical potential is below the barrier height, coherence should still be maintained. For example, calculations based on a simple double-well model show that a BEC of 250-300 atoms would be in the Josephson interaction regime for V barrier ≈ 60 nK (well within the range of our experimental uncertainty), assuming that the BEC is in the ground state [39]. In this regime the tunneling rate is sufficiently fast to maintain coherence, even though the chemical potential is significantly below the barrier.…”

“…We find that the first-order peaks are "locked" at about ±15 µm, independent of t, for the > 1000 images collected in this figure. The progressively declining OD for increasing t indicates an atom lifetime of 500 ± 50 ms, consistent with measured spin-flip rates for this experiment, which are mostly due to technical noise [39]. (b) Data points [same color code as in (a)] show the optical density difference ("OD diff") between the diffraction side-peak maxima and minima, averaged over all the experimental images obtained for each holding time.…”

“…These results should motivate further investigations to elucidate the interplay amongst tunneling, atomic collisions, and effects due to external noise, and their role in maintaining robust spatial coherence [39,[50][51][52]. This experiment also holds promise for the future development of atomic circuits.…”

“…III. Figure 12 shows no such reduction, suggesting instead that indeed the chemical potential is below the barrier, where the tighter confinement within individual lattice sites prevents quasi-condensate phase fluctuations while sufficiently fast tunneling maintains the observed phase coherence [39].…”

Fifteen years of cold matter on the atom chip: promise, realizations, and prospects

“…In this review, we will be focusing on experimental work. Nevertheless, concerning neutral atoms, let us briefly mention here some of the extensive work done on Johnson noise and single-atom decoherence [ 42 , 43 ], electron transport [ 44 – 46 ], CP interactions [ 47 ], decoherence in low dimensions [ 48 ], loss, heating, and decoherence of a many-body system near a surface [ 49 , 50 ], quantum gates [ 51 – 53 ], hybrid devices [ 54 ], superconductors [ 55 – 58 ], exotic materials and geometries affecting everything from Johnson noise to CP forces and vacuum modes [ 59 – 63 ], double-well potentials and matter-wave interferometry [ 64 – 67 ], matter-wave pulse shaping [ 68 ], atomtronics [ 69 – 71 ], and so on. Two more examples consist of a simulator for quantum pumping of electrons in mesoscopic circuits [ 72 ] and the investigation of bosonic superflow [ 73 ].…”